Replace the MD5 hashing of images with xxHash

Note that we only use this for change detection.

The previous implementation invoked `MD5FromReaderFast` that created a MD5 has from 8 64 bytes chunks in the file, which is obviously very fast. The new implementation creates the hash from the entire file and ... seems to be even more effective:

```
name          old time/op    new time/op    delta
HashImage-10    9.45µs ±21%   10.89µs ± 1%     ~     (p=0.343 n=4+4)

name          old alloc/op   new alloc/op   delta
HashImage-10      144B ± 0%        8B ± 0%  -94.44%  (p=0.029 n=4+4)

name          old allocs/op  new allocs/op  delta
HashImage-10      4.00 ± 0%      1.00 ± 0%  -75.00%  (p=0.029 n=4+4)
```

2 files changed, 8 insertions, 7 deletions

diff --git a/resources/image.go b/resources/image.go
index 188f49624..56f0981f2 100644
--- a/resources/image.go
+++ b/resources/image.go
@@ -493,7 +493,7 @@ func (i *imageResource) relTargetPathFromConfig(conf images.ImageConfig) interna
 	}
 
 	h := i.hash()
-	idStr := fmt.Sprintf("_hu%s_%d", h, i.size())
+	idStr := fmt.Sprintf("_hu%d_%d", h, i.size())
 
 	// Do not change for no good reason.
 	const md5Threshold = 100
diff --git a/resources/resource.go b/resources/resource.go
index 19270307e..cc7008e5a 100644
--- a/resources/resource.go
+++ b/resources/resource.go
@@ -26,6 +26,7 @@ import (
 	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/resources/internal"
 
+	"github.com/gohugoio/hugo/common/hashing"
 	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/gohugoio/hugo/common/paths"
 
@@ -307,7 +308,7 @@ type fileInfo interface {
 }
 
 type hashProvider interface {
-	hash() string
+	hash() uint64
 }
 
 var _ resource.StaleInfo = (*StaleValue[any])(nil)
@@ -403,7 +404,7 @@ func (l *genericResource) size() int64 {
 	return l.h.size
 }
 
-func (l *genericResource) hash() string {
+func (l *genericResource) hash() uint64 {
 	if err := l.h.init(l); err != nil {
 		panic(err)
 	}
@@ -628,7 +629,7 @@ type targetPather interface {
 }
 
 type resourceHash struct {
-	value    string
+	value    uint64
 	size     int64
 	initOnce sync.Once
 }
@@ -636,7 +637,7 @@ type resourceHash struct {
 func (r *resourceHash) init(l hugio.ReadSeekCloserProvider) error {
 	var initErr error
 	r.initOnce.Do(func() {
-		var hash string
+		var hash uint64
 		var size int64
 		f, err := l.ReadSeekCloser()
 		if err != nil {
@@ -656,6 +657,6 @@ func (r *resourceHash) init(l hugio.ReadSeekCloserProvider) error {
 	return initErr
 }
 
-func hashImage(r io.ReadSeeker) (string, int64, error) {
-	return helpers.MD5FromReaderFast(r)
+func hashImage(r io.ReadSeeker) (uint64, int64, error) {
+	return hashing.XXHashFromReader(r)
 }